The detection of faults and unconformities is a long-standing problem in petroleum reservoir characterization studies and in the evaluation of exploration oil wells. Faults and unconformities may compartmentalize petroleum reservoirs in ways previously unsuspected. During well completion, an operator may want to avoid faults because they provide permeable conduits to channel water to the borehole. Conversely, an operator may want to preferentially test faults because associated breccias are the only permeable rocks in an otherwise tight section. Angular unconformities, even those with very subtle dip discordances, could be key sequence boundaries.
Dip angle is the angle a bedding plane makes with the earth's surface. Strike angle is the angle of a line formed when a bedding plane intercepts the earth's surface. Strike angle is measured with respect to north, which is zero degrees. Dip direction is the orientation of a line perpendicular to strike and directed down the dip of a given plane. Dip direction is also expressed as a number of degrees wherein north is zero degrees. Bedding plane orientations within a well can be measured from oriented core samples, dipmeter logs, and acoustic or electrical borehole imaging logs.
Oriented core is generally unavailable, because of the difficulties in gathering the cores. Also, because cored intervals are usually much shorter than logged intervals, potential faults and/or unconformities may exist in uncored areas. Because of this, logging methods are most useful for bedding plane detection and measurement.
Dipmeters, such as the stratigraphic high-resolution dipmeter tool (SHDT) introduced by Schlumberger in the early 1980's, provides data on bedding planes of a borehole. The dipmeter has four pads, oriented 90.degree. apart, and each pad contains two electrodes mounted only three centimeters apart. The dipmeter emits an electrical current and measures the resistivity between the two electrodes at a high sampling rate of one sample approximately every one tenth inch. Data is measured by moving the dipmeter through the borehole and recording the electrical resistivity at each sample. Using the sample data, dip angle and direction of bedding planes can be measured within a borehole.
An improved tool for measuring dip angle and direction was introduced by Schlumberger in the mid 1980's. This tool is called a Formation Micro Scanner (FMS) and it operates in a manner similar to a dipmeter. The formation microscanner, however, contains many more electrodes and takes many more measurements which are closer together than the measurements of a dipmeter. The device has arrays of microresistivity sensors set on two, four or eight pads, to give oriented images of the borehole wall, in addition to containing four pads to produce dipmeter measurements. The tool emits a controlled and focused current into the geologic formation, and this current is returned to the tool where the proportion of the current flowing from the dense array of electrodes is recorded as a series of individual curves, representing microconductivity changes such as those due to bedding surfaces or fractures intersecting the well bore. The measurement is shallow with respect to the borehole wall, and has a very good vertical resolution. The tool contains a triaxial accelerometer and three magnetometers to give accurate orientation information and to enable speed corrections to be made to the acquired data.
Data from dipmeters is computer processed to correlate resistivity peaks, and often indicates correlation when the peaks do not actually correlate. Because of this, the tadpole plots produced from dipmeter data commonly show a lot of scatter.
Borehole imaging logs, such as those produced by the Formation Micro Scanner, are the best choice for reliable bedding plane orientations. One advantage of the borehole imaging logs, is that the bedding planes are picked by human analysis, rather than by computer algorithm, and once picked, software then calculates the dip angle and dip direction. This causes the dip angle and direction to be more reliable, and it reduces scatter.
Bedding plane orientations commonly differ above and below faults and unconformities, although this does not always occur. There is need in the art for a method using dip angle and direction to locate faults and unconformities.